The present invention relates to a brushless motor, more particularly relates to a brushless motor in which a rotor can be stopped at a predetermined position and the rotor can be restarted smoothly.
Generally, a brushless motor is provided with a rotor R constituted by a permanent magnet, a plurality of excitation coils C (driving coils) positioned in the circumference of the rotor R, a rotor position detecting member S such as a magnetosensitive element, and a commutation circuit for switching the electric current for the excitation coils C based on the detection signal of the rotor position detecting member S.
The position of the magnetic poles of the rotor R relative to the other stationary members varies as the rotor R rotates. Accordingly, if such an electric motor of this kind is once driven, it is impossible to stop the rotor R so that the magnetic poles come to desired positions relative to the excitation coils C.
In such a conventional electric motor of this kind, therefore, the rotor R may sometimes stop at a position where the magnetic poles of the rotor R directly face the respective excitation coils C, as shown in FIG. 9. In such a case, even if the motor is driven to restart, the motor may fall into a trough of torque in relation with the revolving angle of the rotor so that smooth restart can not be expected. This applies even the case where a rotor position detecting member S such as a magnetosensitive element or a magnetic-pole saturation element is provided in order to properly excite the excitation coils C. Further, when the rotor R is in such a stoppage position as shown in FIG. 9, the magnetic-pole detecting element cannot operate satisfactorily or operates erroneously to make the rotor rotate reversely.
In order to solve such a problem in the prior art, therefore, the same applicant as that of this application has proposed a brushless motor in which the rotor can be stopped at a predetermined position relative to the excitation coils whenever it is stopped, namely at a position where no dead point is caused in starting operation, and in which a rotor position detecting member is able to operate properly, in U.S. patent application Ser. Nos. 245,3362 and 245,363 corresponding to Japanese Patent Unexamined Publication Nos. 64-81633 and 64-81651.
In order to attain the above object, the brushless motor disclosed in the above Japanese Patent Unexamined Publication No. 64-81651 has the following features. That is, in a brushless motor comprising a rotor constituted by a permanent magnet having a plurality of circumferentially equally magnetized magnetic poles, a plurality of excitation coils equidistantly disposed around the rotor, a rotor position detecting member such as a magneto-sensitive element, and a commutation circuit for switching the electric current for the excitation coils based on the detection signal of the rotor position detecting member, the number of the magnetic poles of the rotor is selected so that the number of the magnetic poles of the rotor located in an area between the respective center lines of two adjacent excitation coils is an odd number not smaller than three and the width of each excitation coil is selected so that an angle .alpha. formed between lines connecting the circumferential opposite ends of each excitation coil to a center of the rotor is made equal to an angle .beta. formed between lines connecting the circumferential opposite ends of each magnetic pole of the rotor to the center of the rotor.